Results 1 - 10 of 5096
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[en] Microwave emissions were monitored from the HR 1099 binary system in response to large radio wavelength emissions from that system. HR 1099 is in the group of RS Canum Venaticorum type binary stars which are similar to our sun in mass, temperature and luminosity. Gigantic flare explosions result from stellar spots which may cover as much as 20 percent of the surface of one of the binary members and which may become unstable due to the proximity of the companion star. These large-scale stellar spots may provide additional insight into the mechanisms at work in our own sun whose activity is much less. (TI)
[en] Two large northern polar crown prominences that erupted on 2010 April 13 and 2010 August 1 were analyzed using images obtained from the Extreme UltraViolet Imager on the twin Solar Terrestrial Relations Observatory spacecraft. Several features along the prominence legs were reconstructed using a stereoscopic reconstruction technique developed by us. The three-dimensional changes exhibited by the prominences can be explained as an interplay between two different motions, namely helical twist in the prominence spine, and overall non-radial equatorward motion of the entire prominence structure. The sense of twist in both the prominences is determined from the changes in latitudes and longitudes of the reconstructed features. The prominences are observed starting from a few hours before the eruption. Increase in height before and during the eruption allowed us to study the kinematics of the prominences in the two phases of eruption, the slow-rise and the fast-eruptive phase. A constant value of acceleration was found for each reconstructed feature in each phase, but it showed a significant change from one leg to the other in both the prominences. The magnitude of acceleration during the eruptive phase is found to be commensurate with the net effect of the two motions stated above.
[en] Three novel classes of exact solutions of the generalized Grad--Shafranov equation for helically symmetric magnetohydrodynamic (MHD) equilibria are presented. The first two classes may be applied to helical MHD equilibria for plasma confined between two coaxial cylinders, while the third one to the modeling of helicoidal magnetic fields and flows in several recently observed astrophysical jets. The same solutions can be also used for the testing of sophisticated numerical codes. It is also shown that all helically symmetric MHD equilibria can be treated by the same general method which is employed to generate exact MHD solutions for systems possessing an ignorable coordinate in a system of three orthogonal basis vectors, although in the case of helical symmetry an orthogonal ignorable coordinate does not exist, contrary to what happens in the well-known cases of axial and translational symmetries
[en] High-degree solar mode frequencies as measured by ring diagrams are known to change in the presence of the strong magnetic fields found in active regions. We examine these changes in frequency for a large sample of active regions analyzed with data from the Michelson Doppler Imager on board the Solar and Heliospheric Observatory spacecraft, spanning most of solar cycle 23. We confirm that the frequencies increase with increasing magnetic field strength, and that this dependence is generally linear. We find that the dependence is slightly but significantly different for active regions with different sunspot types.
[en] The magnetic field of the umbrae is sometimes found to be saturated in the magnetograms taken by the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO). It is suggested that the combination of the low intensity of sunspot umbrae and the limitation of the 15-bit onboard numerical data acquisition leads to this saturation. In this paper, we propose to use the MDI's intensity data to correct this saturation. This method is based on the well-established relationship between the continuum intensity and the magnetic field (the so-called I-B relationship). A comparison between the corrected magnetic field and the data taken by the Stokes-Polarimeter of the Solar Optical Telescope (SOT/SP) onboard Hinode shows a reasonable agreement, suggesting that this correction is effective. (research papers)
[en] We study possible interrelations between the 300-year record of the yearly sunspot numbers and the solar inertial motion (SIM) using the recently developed technique of synchronization analysis. Phase synchronization of the sunspot cycle and the SIM is found and statistically confirmed in three epochs (1734-1790, 1855-1875 and 1907-1960) of the whole period 1700-2000. These results give quantitative support to the hypothesis that there is a weak interaction between the solar activity and the SIM
[en] Complete text of publication follows. There is increasing evidence that various manifestations of solar activity are non-axisymmetric and mainly occur in two preferred longitude ranges, so called active longitudes. We have earlier analyzed the longitudinal occurrence of solar X-ray flares observed by GOES satellites using a specially developed dynamic, differentially rotating coordinate system. In this frame, the longitude distribution shows two persistent preferred longitudes separated by about 180 degrees whose strength alternates in time, similarly to the so called flip-flop phenomenon. Here we make an improved statistical analysis to find the globally best fitting values for the parameters describing the differential rotation of active longitudes. We find that the revised analysis gives a more consistent set of parameters, e.g., for the different classes of X-ray flares. Also, the improved parameters yield a higher level of non-axisymmetry for the longitudinal distribution, thus increasing evidence for the existence of active longitudes.
[en] The maximum amplitude (Rm) of a solar cycle, in the term of mean sunspot numbers, is well-known to be positively correlated with the preceding minimum (Rmin). So far as the long term trend is concerned, a low level of Rmin tends to be followed by a weak Rm, and vice versa. We found that the evidence is insufficient to infer a very weak Cycle 24 from the very low Rmin in the preceding cycle. This is concluded by analyzing the correlation in the temporal variations of parameters for two successive cycles. (letters)
[en] Complete text of publication follows. The onset of solar cycle 24 was once announced and then cancelled. The prolonged cycle 23 has delayed the onset of solar cycle 24 by two more years than the predicted. Solar magnetic fields and the Sun-Earth connection related parameters covariant with the solar cyclic change of sunspot number (SSN) revealed low activities during the maximum years of last solar cycle 23 and are currently showing unprecedented level of low activities at the solar minimum years of solar cycle 23 ending period. The strangeness of unprecedented activity levels of most parameters may not be sufficient to draw invincible conclusions with only three cycle data. However, the SSN has been recorded for four hundred years with modern telescopic observation. The historians and the artists recorded how severely cold during the Maunder and Dalton minimums, when the SSN at the solar cycle maximum years is very low. The solar cycle length has a good reverse correlation with the global surface temperature [Friis-Christensen and Lassen, 1991]. Thus, the reality of those relationships with global climate could be tested during the coming solar cycle 24. Then, the solar cycle 24 will be a stage for arguing between two scenarios of the anthropogenic global warming and the solar activity driven climate.
[en] We investigate the cyclic evolutionary behavior of CME accelerations for accelerating and decelerating CME events in cycle 23 from 1997 January to 2007 December. It is found that the absolute values of semiannual mean accelerations of both accelerating and decelerating CME events roughly wax and wane in a cycle, delaying the sunspot cycle in time phase. We also investigate the semiannual number of CMEs with positive and negative acceleration and find that there are more decelerating CME events than accelerating CME events during the maximum period of a cycle (about three years), but there are more accelerating CME events than decelerating CME events during the rest of the time interval of the cycle. Our results seem to suggest that the different driving mechanisms may be acting accelerate and decelerate CME events: for accelerating CME events, the propelling force (Fp) statistically seems to play a significant role in pushing CMEs outward; for decelerating CME events, the drag (Fd) statistically seems to play a more effective role in determining CME kinematic evolution in the outer corona. During the maximum period of a cycle, because of the V2 dependence, Fd is generally stronger; because of the magnetic field dependence, Fp is also generally stronger. Thus, the absolute values of both the negative and positive accelerations are generally larger during that time. Because of the V2 dependence, Fd may be more effective during the maximum period of a cycle. Hence, there are more decelerating CME events than accelerating CME events during that time. During the minimum time interval of a cycle, CMEs have relatively small speeds, and Fp may be more effective. Therefore, there are more accelerating CME events than decelerating CME events during that time.